The present disclosure relates to a sheet post-processing device that performs post-processing including a punch hole forming process, a binding process, and so on with respect to a sheet such as a paper sheet on which an image has been formed by an image forming apparatus such as a copy machine, facsimile, or a printer, and an image forming system provided with the same.
Conventionally, there is used a paper sheet post-processing device that is capable of executing post-processing including a binding process of stacking a plurality of paper sheets (sheets) on each of which an image has been formed by an image forming apparatus such as a copy machine or a printer and binding together a bundle of the thus stacked paper sheets with a staple(s), a punch hole forming process of forming punch holes (perforations) therethrough by using a punch hole forming device, and so on.
In such a paper sheet post-processing device, the punch hole forming device is configured to form a plurality of punch holes through a paper sheet along its end edge not in a conveyance direction thereof but in a direction orthogonal to the conveyance direction. This is because of the following reasons. That is, in a case where punch holes are formed through a paper sheet along its end edge in a conveyance direction thereof, there arises a need for temporarily stopping conveyance of the paper sheet, causing a fear of impairing productivity (processing efficiency). Also, in an image forming apparatus that is capable of printing on an A3-size paper sheet, lateral conveyance (lateral paper passing) of an A4-size paper sheet is enabled, and hence there has been no demand to form punch holes through a paper sheet along its end edge in a conveyance direction thereof.
In recent years, however, in response to a request for a space-saving image forming apparatus, an image forming apparatus of a type that performs longitudinal conveyance (longitudinal paper passing) of an A4-size paper sheet has been on the market. Correspondingly therewith, there is, therefore, also a rising demand to form punch holes through a paper sheet along its end edge in a conveyance direction thereof.
As a method for forming a plurality of punch holes in a paper sheet conveyance direction, it is known to use a post-processing device that changes perforation positions with respect to a center line of a sheet so that, in performing a folding process with respect to the sheet after being subjected to a perforation process, with the sheet folded, two of four perforations formed therethrough coincide with the other two. Furthermore, a sheet post-processing device that forms punch holes without stopping conveyance of a sheet being conveyed also is known, in which a perforation device and an end part detection unit that detects a side end of the sheet are made movable in a direction orthogonal to a conveyance direction of the sheet so that punch holes are formed at positions corresponding to a size of the sheet conveyed thereto.
By the way, normally, before being ejected on an ejection tray, a paper sheet is subjected to stiffening by a stiffening member so as not to be rolled up. Due to stiffening, however, as shown in FIG. 6, each end part of a subsequent paper sheet 201 in a width direction thereof (a direction perpendicular to a paper sheet conveyance direction) is disposed more inwardly than each end part of a paper sheet 200 in a width direction thereof, which has already been ejected.